Field of the Invention
The present invention relates to a sheet conveying apparatus installed in an image forming apparatus such as a copying machine, a fax machine, and a printer in order to allow a sheet to be stably conveyed, and to an image forming apparatus provided with the sheet conveying apparatus.
Description of the Related Art
If a sheet in an image forming apparatus is skew-fed while being conveyed, or is misaligned in a width direction of the sheet perpendicular to the sheet conveyance direction, the apparatus ends up by forming an image misaligned with respect to the sheet. In order to deal with the problem, a sheet aligning apparatus is provided immediately in front of a transfer portion so as to correct skew feeding of a sheet and adjust the position of a sheet in its width direction. As an example of the sheet aligning apparatus, there is a sheet registration device for correcting a misalignment of a sheet being conveyed, with respect to a side edge of the sheet (see U.S. Pat. No. 6,273,418).
A sheet aligning apparatus that corrects skew feeding according to the side registration reference includes an abutting reference member along a direction in which a sheet is conveyed (sheet conveyance direction), and a pair of skew feeding correcting rollers above the sheet conveying path.
The abutting reference member has a reference surface parallel to the sheet conveyance direction. The number of pairs of skew feeding correcting rollers provided is at least two, and they are arranged along the reference surface in the sheet conveyance direction. While a sheet is being conveyed, the pairs of skew feeding correcting rollers skew-feed the sheet toward the abutting reference member and make a side edge of the sheet abut against the reference surface so that the side edge will extend along the reference surface. The skew of the sheet in the sheet conveyance direction is thus corrected. Further, since the reference surface can fix the position of a side edge of a sheet with respect to a direction perpendicular to the sheet conveyance direction, the misalignment of a sheet in the width direction can be corrected based on the reference surface.
If the force of pressing the reference surface by a sheet (the force of drawing a sheet by the pairs of skew feeding correcting rollers) is excessively large when a side of the sheet is set to abut against the reference member in the sheet aligning apparatus, the sheet bends and a sheet jam or deterioration of correction accuracy occurs.
In order to solve the above problem, there is proposed an apparatus with existing pairs of skew feeding correcting rollers that adjusts the force of pressing the reference surface by a sheet by changing the nip pressure of a pair of skew feeding correcting rollers according to the thickness of a sheet, in correcting skew feeding with the existing pairs of rollers (see U.S. Pat. No. 5,253,862). This apparatus can correct skew feeding without bending a sheet when drawing a side edge of the sheet to the reference surface.
The existing method of correcting skew feeding of a sheet according to the side registration reference by adjusting the nip pressure of the skew feeding is disadvantageous in that there is difficulty in setting the position of a sheet in a stable state in correcting skew feeding, although the force of pressing the abutting reference member can be adjusted. In particular, when a sheet to be conveyed is such a paper as a coated paper with a basis weight of smaller than 80 gsm, which is super thin and has a small rigidity or when a sheet has a reduced rigidity under high-temperature and high-humidity environments, the following problems may occur.
The number of pairs of skew feeding correcting rollers provided is two or more, and they are arranged in the sheet conveyance direction. When an upstream pair of the skew feeding correcting rollers that first conveys a sheet starts conveying a sheet, the sheet is rotated and is caused to abut against the reference surface of the abutting reference member at a side edge of the sheet with a bent occurring at the side edge. Although a sheet with a large rigidity is not easily bent, a sheet with a small rigidity is bent and passed to a next pair of skew feeding correcting rollers. Accordingly, the difference in the degree of bending generated in the sheet conveyance direction can result into insufficient correction of skew feeding, or the bending can lead to buckling (breaking) and a paper jam may be caused.